Power transmission devices with active torque couplings

ABSTRACT

A torque transfer mechanism having a multi-plate friction clutch connecting a pair of rotary members and a electrohydraulic clutch actuator for controlling engagement of the friction clutch. The clutch actuator includes a hydraulic pump, a hydraulically actuated rotary operator, and a thrust mechanism. The hydraulic pump draws low pressure fluid from a sump and selectively delivers high pressure fluid to a series of actuation chambers and return chambers defined between coaxially aligned first and second components of the rotary operator. The magnitude of the fluid pressure delivered to the actuation chamber controls angular movement of the second component relative to the first component for energizing the thrust mechanism. The thrust mechanism applies a clutch engagement force on the friction clutch, thereby transferring drive torque from the first rotary member to the second rotary member. An electrohydraulic control system regulates the fluid pressure delivered to the actuation and return chambers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/771,664 filed on Feb. 4, 2004.

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle and/or the left and rightwheels of an axle assembly. More particularly, the present invention isdirected to a power transmission device for use in motor vehicledriveline applications having a torque transfer mechanism equipped witha clutch actuator that is operable for controlling actuation of amulti-plate friction clutch.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being incorporated into vehiculardriveline applications for transferring drive torque to the wheels. Inmany vehicles, a power transmission device is operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelyand/or automatically transferring drive torque from the primarydriveline to the secondary driveline to establish a four-wheel drivemode of operation. For example, the torque transfer mechanism caninclude a dog-type lock-up clutch that can be selectively engaged forrigidly coupling the secondary driveline to the primary driveline toestablish a “part-time” four-wheel drive mode. When the lock-up clutchis released, drive torque is only delivered to the primary driveline forestablishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with an adaptively controlled transfer clutch inplace of the lock-up clutch. The transfer clutch is operable forautomatically directing drive torque to the secondary wheels, withoutany input or action on the part of the vehicle operator, when tractionis lost at the primary wheels for establishing an “on-demand” four-wheeldrive mode. Typically, the transfer clutch includes a multi-plate clutchassembly that is installed between the primary and secondary drivelinesand a clutch actuator for generating a clutch engagement force that isapplied to the clutch assembly. The clutch actuator can be apower-operated device that is actuated in response to electric controlsignals sent from an electronic controller unit (ECU). The electriccontrol signals are typically based on changes in current operatingcharacteristics of the vehicle (i.e., vehicle speed, interaxle speeddifference, acceleration, steering angle, etc.) as detected by varioussensors. Thus, such “on-demand” transfer clutch can utilize adaptivecontrol schemes for automatically controlling torque distribution duringall types of driving and road conditions. Such adaptively controlledtransfer clutches can also be used in association with a centerdifferential operably installed between the primary and secondarydrivelines for automatically controlling interaxle slip and torquebiasing in a full-time four-wheel drive application.

A large number of adaptively controlled transfer clutches have beendeveloped with an electromechanical clutch actuator that can regulatethe amount of drive torque transferred to the secondary driveline as afunction of the electric control signal applied thereto. In someapplications, the transfer clutch employs an electromagnetic clutch asthe power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024discloses a electromagnetic coil that is incrementally activated tocontrol movement of a ball-ramp drive assembly for applying a clutchengagement force to the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 62-18117 discloses a transfer clutchequipped with an electromagnetic clutch actuator for directlycontrolling actuation of the multi-plate clutch pack assembly. As analternative, the transfer clutch can employ an electric motor and amechanical drive assembly as the power-operated clutch actuator. Forexample, U.S. Pat. No. 5,323,871 discloses a transfer clutch equippedwith an electric motor that controls rotation of a sector plate which,in turn, controls pivotal movement of a lever arm that is operable forapplying the clutch engagement force to the multi-plate clutch assembly.Likewise, Japanese Laid-open Patent Application No. 63-66927 discloses atransfer clutch which uses an electric motor to rotate one cam plate ofa ball-ramp operator for engaging the multi-plate clutch assembly.Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose atransfer clutch having an electric motor which drives a reductiongearset for controlling movement of a ball screw operator and aball-ramp operator which, in turn, apply the clutch engagement force tothe clutch assembly.

In contrast to the electromechanical clutch actuators discussedpreviously, it is also well known to equip the transfer clutch with anelectro-hydraulic clutch actuator. For example, U.S. Pat. Nos. 4,862,769and 5,224,906 generally disclose use of an electric motor or solenoid tocontrol the fluid pressure exerted by an apply piston on a multi-plateclutch assembly. In addition, U.S. Pat. No. 6,520,880 discloses ahydraulic actuation system for controlling the fluid pressure suppliedto a hydraulic motor arranged which is associated with a differentialgear mechanism in a drive axle assembly.

While many adaptive clutch actuation systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size and weight of the friction clutch components and theelectrical power requirements of the clutch actuator needed to providethe large clutch engagement loads make many systems cost prohibitive foruse in most four-wheel drive vehicle applications. In an effort toaddress these concerns, new technologies are being developed for use inpower-operated clutch actuator applications.

SUMMARY OF THE INVENTION

Thus, its is an objective of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a unique electrohydraulically-operated clutchactuator that is operable to control adaptive engagement of amulti-plate clutch assembly.

As a related objective of the present invention, the torque transfermechanism is well-suited for use in motor vehicle driveline applicationsto adaptively control the transfer of drive torque between first andsecond rotary members.

According to each preferred embodiment of the present invention, atorque transfer mechanism and an electrohydraulic control system aredisclosed for adaptively controlling the transfer of drive torquebetween first and second rotary members in a power transmission deviceof the type used in motor vehicle driveline applications. The torquetransfer mechanism includes a multi-plate friction clutch that isoperably disposed between the first and second rotary members, and aclutch actuator that is operable for generating and applying a clutchengagement force on the friction clutch. The clutch actuator includes arotary operator and a thrust mechanism. The electrohydraulic controlsystem functions to deliver pressurized fluid to a plurality ofactuation chambers defined between coaxially aligned first and secondcomponents of the rotary operator. During operation, the magnitude ofthe fluid pressure delivered to the actuation chambers controls theangular movement of the second component relative to the firstcomponent. Such relative angular movement controls actuation of thethrust mechanism for controlling the magnitude of the compressive clutchengagement force applied to the friction clutch, thereby controlling thetransfer of drive torque from the first rotary member to the secondrotary member.

According to another feature of the present invention, theelectrohydraulic control system includes an electric motor for driving afluid pump, vehicle sensors for detecting various operatingcharacteristics of the motor vehicle, and an electronic control unit(ECU) for receiving input signals from the vehicle sensors. The ECUcontrols actuation of the motor and one or more control valves foradaptively controlling the fluid pressure supplied to the actuationchambers. In addition, a pressure sensor provides a pressure signal tothe ECU that is indicative of the fluid pressure in the actuationchambers.

According to yet another feature of the present invention, the first andsecond components of the rotary operator further define a plurality ofreturn chambers that are located between the actuation chambers. Inoperation, a control valve is operable for selectively supplyinghydraulic fluid to either the actuation chambers or the return chambersto precisely control relative angular movement between the first andsecond components of the rotary operator.

The torque transfer mechanism of the present invention is well-suitedfor use in a power transmission device for adaptively controlling thedrive torque transferred between a primary driveline and a secondarydriveline. According to one preferred application, the powertransmission device of the transfer case with the torque transfermechanism arranged torque transfer between the primary and secondarydrivelines. the power transmission device is a power take-off unit or adrive axle assembly with the torque transfer mechanism arranged toprovide on-demand torque transfer from the primary driveline to thesecondary driveline. According to yet another preferred application, thepower transmission device is a drive axle assembly with the torquetransfer mechanism arranged to control speed differentiation and torquedistribution across a differential unit.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives, features and advantages of the present inventionwill become apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and the accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of a four-wheel drive vehicle equippedwith a power transmission device according to the present invention;

FIG. 2 is a schematic illustration of the power transmission deviceequipped with a torque transfer mechanism embodying the inventiveconcepts of the present invention;

FIGS. 3 and 3A are sectional views of the torque transfer mechanismconstructed in accordance with a preferred embodiment of the presentinvention;

FIG. 4 is a sectional view of the rotary operator associated with thetorque transfer mechanism shown in FIGS. 3 and 3A;

FIG. 5 is a schematic diagram of the electrohydraulic control circuitassociated with the torque transfer mechanism of the present invention;

FIGS. 6 through 8 are schematic diagrams of alternative electrohydrauliccontrol systems adapted for use with the torque transfer mechanism ofthe present invention;

FIG. 9 is a schematic illustration of an alternative power transmissiondevice available for use with the drivetrain shown in FIG. 1;

FIG. 10 is a schematic illustration of another alternative embodiment ofa power transmission device according to the present invention;

FIG. 11 illustrates an alternative drivetrain arrangement for afourwheel drive motor vehicle equipped with another power transmissiondevice embodying the present invention;

FIGS. 12 through 15 schematically illustrate different embodiments ofthe power transmission device shown in FIG. 11;

FIG. 16 is an illustration of another drivetrain arrangement for afour-wheel drive vehicle equipped with a power transmission deviceembodying the present invention; and

FIG. 17 is a schematic illustration of an alternative construction forthe power transmission device shown in FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferred froma first rotary member to a second rotary member. The torque transfermechanism finds particular application in power transmission devices foruse in motor vehicle drivelines such as, for example, a torque transferclutch in a transfer case, a power take-off unit or an in-line torquecoupling, a torque biasing clutch associated with a differential unit infull-time transfer cases or power take-off unit or in a drive axleassembly, or any other possible torque transfer application. Thus, whilethe present invention is hereinafter described in association withparticular power transmission devices for use in specific drivelineapplications, it will be understood that the arrangements shown anddescribed are merely intended to illustrate embodiments of the presentinvention.

With reference to FIG. 1, a schematic layout of a vehicular drivetrain10 is shown to include a powertrain 12, a first or primary driveline 14driven by powertrain 12, and a second or secondary driveline 16.Powertrain 12 includes an engine 18 and a multi-speed transaxle 20arranged to normally provide motive power (i.e., drive torque) to a pairof first wheels 22 associated with primary driveline 14. Primarydriveline 14 further includes a pair of axleshafts 24 connecting wheels22 to a differential unit 25 associated with transaxle 20. Secondarydriveline 16 includes a power take-off unit (PTU) 26 driven by theoutput of transaxle 20, a propshaft 28 driven by PTU 26, a pair ofaxleshafts 30 connected to a pair of second wheels 32, and a drive axleassembly 34 that is operable to selectively transfer drive torque frompropshaft 28 to axle halfshafts 30.

Drive axle assembly 34 is a power transmission device according to onepreferred embodiment of the present invention. In particular, drive axleassembly 34 is shown schematically in FIG. 2 to include a housingassembly 36 which encloses a torque transfer mechanism 38 and adifferential unit 40. Torque transfer mechanism 38 functions toselectively transfer drive torque from propshaft 28 to an inputcomponent of differential unit 40. Specifically, the torque transfermechanism, hereinafter referred to as torque coupling 38, includes aninput shaft 42 driven by propshaft 28, a pinion shaft 44, a transferclutch 46 operably connected between input shaft 42 and pinion shaft 44,and a clutch actuator 48 for engaging transfer clutch 46.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system forpermitting a vehicle operator to select between a two-wheel drive mode,a locked (“part-time”) four-wheel drive mode, and an adaptive(“on-demand”) four-wheel drive mode. In this regard, transfer clutch 46can be selectively engaged for transferring drive torque from inputshaft 42 to pinion shaft 44 for establishing both of the part-time andon-demand four-wheel drive modes. The power transfer system includes anelectrohydraulic control system 50 for selectively actuating clutchactuator 48, vehicle sensors 52 for detecting certain dynamic andoperational characteristics of the motor vehicle, a mode selectmechanism 54 for permitting the vehicle operator to select one of theavailable drive modes, and an electronic control unit (ECU) 56 forcontrolling operation of the components associated with electrohydrauliccontrol system 50 which, in turn, controls actuation of clutch actuator48 in response to input signals from vehicle sensors 52 and modeselector 54.

Referring primarily to FIGS. 2 and 3, transfer clutch 46 generallyincludes a first clutch member 60 driven by input shaft 42, a secondclutch member 62 driving pinion shaft 44, and a multi-plate clutch pack64 of alternately interleaved clutch plates installed between inputshaft 42 and pinion shaft 44. As shown in this particular arrangement,first clutch member 60 is a clutch drum fixed for rotation with inputshaft 42 and second clutch member 62 is a clutch hub fixed (i.e.,splined) for rotation with pinion shaft 44. Pinion shaft 44 drives apinion gear 66 meshed with a ring gear 68 which, in turn, drivesdifferential unit 40. Differential unit 40 includes a carrier 70 drivenby ring gear 68, a pair of pinion gears 72 rotatably supported on pinionposts 74 fixed to carrier 70, and a pair of side gears 76. Side gears 76are meshed with both pinion gears 74 and are coupled for rotation with acorresponding one of axleshafts 30.

Referring now to FIGS. 3 and 3A, a preferred construction for torquecoupling 38 will now be described in greater detail. Torque coupling 38includes a case assembly 78 that is mounted in or forms part of housing36 of drive axle assembly 34. A bearing assembly 80 supports input shaft42 for rotation relative to case assembly 78. In addition, input shaft42 is shown to include an integral end plate 82 that is rigidly secured(i.e., welded) to clutch drum 60. An end plate segment 84 of drum 60 isrotatively supported by a bearing assembly 86 from case assembly 78.Pinion shaft 44 has a first end rotatably supported by a bushing orbearing assembly 88 in a central bore formed in input shaft 42 while itssecond end extends out of end plate segment 84 of drum 60 and isrotatably supported from case assembly 78 by a bearing assembly 90.Clutch actuator 48 generally includes a rotary operator 92, a thrustmechanism 94, and an apply plate 96. Apply plate 96 is secured (i.e.,splined) for rotation with drum 60 of transfer clutch 46.

As will be detailed, clutch actuator 48 is operable for generating andexerting a compressive clutch engagement force on clutch pack 64. Suchengagement of clutch pack 64 causes rotary power (“drive torque”) to betransferred from input shaft 42 to pinion shaft 44. Specifically, clutchactuator 48 is operable for controlling axial movement of apply plate 96and thus, the magnitude of the clutch engagement force applied to clutchpack 64. In particular, apply plate 96 is axially moveable relative toclutch pack 64 between a first or “released” position and a second or“locked” position. With apply plate 96 in its released position, aminimum clutch engagement force is exerted on clutch pack 64 such thatvirtually no drive torque is transferred from input shaft 42 throughtransfer clutch 46 to pinion shaft 44, thereby establishing thetwo-wheel drive mode. In contrast, movement of apply plate 96 to itslocked position causes a maximum clutch engagement force to be appliedto clutch pack 64 such that pinion shaft 44 is, in effect, coupled forcommon rotation with input shaft 42, thereby establishing part-timefour-wheel drive mode. Accordingly, controlling the position of applyplate 96 between its released and locked positions permits adaptiveregulation of the amount of drive torque transferred from input shaft 42to pinion shaft 44, thereby establishing the on-demand four-wheel drivemode.

Rotary operator 92 includes a first or reaction ring 100 that isconcentrically aligned with a second or “actuator” ring 102. The ringsare retained in an annular chamber for 104 defined between end plate 82and a retainer plate 106. While not shown, retainer plate 106 is securedby a plurality of bolts to end plate 82 which also extend throughmounting holes 108 in reaction ring 100. As such, reaction ring 100 isfixed for common rotation with input shaft 42.

As best seen from FIG. 4, reaction ring 100 includes a cylindrical bodysegment 110 and a plurality of radially inwardly projecting lugs 112which define a complimentary number of longitudinally extending channels114 therebetween. Likewise, actuator ring 102 has a cylindrical bodysegment 116 and a plurality of separator plates 118 that are retained inlongitudinal slots 120 formed in body segment 116. An outer edge surface122 of each separator plate 118 is aligned to be in sliding engagementwith an inner wall surface 124 of channels 114. A seal strip 126 may beattached to edge surface 122 to provide a sliding fluid-tight interfacebetween separator plates 118 and body segment 110 of reaction ring 100.Likewise, a terminal edge surface 128 of each lug 112 is aligned to bein sliding engagement with an outer wall surface 130 of actuator ring102. Each separator plate 118 defines an actuation chamber 132 and areturn chamber 134 between adjacent pairs of lugs 112. As such, aplurality of circumferentially-spaced alternating actuation chambers 132and return chambers 134 are established in association with rotaryoperator 92.

To provide means for supplying hydraulic fluid from electrohydrauliccontrol system 50 to actuation chambers 132, a first flow path is formedin input shaft 42 and its end plate segment 82. The first flow passageincludes an annular chamber 140 which communicates with a series ofcircumferentially-spaced flow passages 142 having ports 144 in fluidcommunication with actuation chambers 132. Similarly, a second flow pathprovides means for supplying hydraulic fluid from control system 50 toreturn chambers 134. This second flow path includes an annular chamber146 which communicates with a series of circumferentially-spaced flowpassages 148 having ports 150 in fluid communication with returnchambers 134. As will be detailed, increasing the fluid pressuredelivered through ports 144 to actuation chambers 132 while decreasingthe fluid pressure delivered through ports 150 to return chambers 134causes actuator ring 102 to move (i.e., index) in a first rotarydirection (i.e., counterclockwise) relative to reaction ring 100 forcausing thrust mechanism 94 to urge apply plate 96 to move toward itslocked position. In contrast, decreasing the fluid pressure in actuationchambers 132 and increasing the fluid pressure in return chambers 134causes actuator ring 102 to index in a second rotary direction (i.e.,clockwise) relative to reaction ring 100 for causing thrust mechanism 94to permit apply plate 96 to move toward its released position.

With continued reference primarily to FIG. 3A, thrust mechanism 94 isshown to be a ball ramp unit having a first cam member 152, a second cammember 154, and rollers, such as balls 156. First cam member 152 isfixed via a spline connection 158 for rotary movement with actuator ring102. Likewise, second cam member 154 is fixed to apply plate 96 forcommon rotation with drum 60 and input shaft 42. Each ball 156 isdisposed in a cam channel defined between a cam track 170 formed infirst cam member 152 and a corresponding cam track 172 formed in secondcam member 154. Preferably, a plurality of cam channels are providedbetween first cam member 152 and second cam member 154 with cam tracks170 and 172 configured as oblique sections of a helical torus. However,balls 156 and cam tracks 170 and 172 may be replaced with alternativecam components and/or ramp configurations that function to cause axialdisplacement of second cam member 154. In any arrangement, the loadtransferring components can not be self-locking or self-engaging so asto permit fine control of the translational movement of apply plate 96(via second cam member 154) for precise control of the engagementcharacteristics (i.e., torque transfer) of transfer clutch 46. As seen,a thrust bearing 176 is located between end plate 82 and first cammember 152.

Ball ramp unit 94 further includes a torsional return spring 178 that isoperably disposed between cam members 152 and 154. Return spring 178functions to angularly bias cam members 152 and 154 to return to a“retracted” position for de energizing ball ramp unit 94. Such angularmovement of the cam members to the retracted position due to the biasingof return spring 178 results in angular movement of actuator ring 102relative to reaction ring 102 in the second angular direction toward afirst or “low pressure” position and translational movement of applyplate 96 toward its released position. With actuator ring 102 in its lowpressure position (as shown in FIG. 4), ball ramp unit 94 isde-energized and apply plate 96 is in its released position so as toexert a predetermined minimum clutch engagement force on clutch pack 64for releasing engagement of transfer clutch 46.

Electrohydraulic control system 50 is operable to supply high pressurefluid to actuation chambers 132 for causing actuator ring 102 to rotaterelative to reaction ring 100 in the first direction from its lowpressure position toward a second or “high pressure” position. Suchmovement of actuator ring 102 results in corresponding relative angularmovement between cam members 152 and 154 from the retracted positiontoward a second or “extended” position for energizing ball ramp unit 94.Accordingly, the profile of cam tracks 170 and 172 establishes theresultant amount of translational movement of second cam member 154required to cause corresponding axial movement of apply plate 96 fromits released position toward its locked position. When actuator ring 102is in its high pressure position, ball ramp unit 94 is fully energizedand apply plate 96 is located in its locked position such that themaximum clutch engagement force is exerted on clutch pack 64 for fullyengaging transfer clutch 46. Furthermore, electrohydraulic controlsystem 50 is operable to supply high pressure fluid to return chambers134 and vent actuation chambers 132 for causing actuator ring 102 torotate relative to reaction ring 100 in the second direction from itshigh pressure position. Such angular movement of actuator ring 102results in corresponding relative angular movement between cam members152 and 154 from the extended position toward the retracted position forreleasing ball ramp unit 94. As such, apply plate 96 is caused to movefrom its locked position toward its released position for releasingengagement of transfer clutch 46.

With apply plate 96 in its released position, virtually no drive torqueis transferred from input shaft 42 to pinion shaft 44 through torquecoupling 38 so as to effectively establish the two-wheel drive mode. Incontrast, location of apply plate 96 in its locked position results in amaximum amount of drive torque being transferred to pinion shaft 44 forcoupling pinion shaft 44 for common rotation with input shaft 42,thereby establishing the part-time four-wheel drive mode. Accordingly,controlling the position of apply plate 96 between its released andlocked positions permits variable control of the amount of drive torquetransferred from input shaft 42 to pinion shaft 44 for establishing anon-demand four-wheel drive mode. Thus, the magnitude of the fluidpressure supplied to actuation chambers 132 and return chambers 134controls the angular position of actuator ring 102 relative to reactionring 100, thereby controlling actuation of ball ramp unit 94 and theresulting movement of apply plate 96 between its released and lockedpositions.

Referring to FIGS. 3 and 5, the components associated withelectrohydraulic control system 50 will now be described. The basiccomponents of control system 50 include a fluid pump 190 operable todraw fluid from a fluid source or sump 192 within casing 78, an electricmotor 194 driving pump 190, an electronic variable orifice (EVO) valve196, and a directional control valve 198. Control system 50 furtherincludes a temperature sensor 200, a pressure sensor 202 and aspring-type pressure relief valve (PRV) 204. Motor 194 is a low powerunit operable for causing pump 190 to draw fluid from sump 192 through afirst flow path 206. The magnitude of the line pressure discharged frompump 190 is delivered to an inlet of EVO valve 196 through a second flowpath 208. However, PRV 204 functions to limit the line pressuredelivered to the inlet of EVO valve 196 to a predefined maximum fluidpressure value. EVO valve 196 includes a moveable valve element thatfunctions to vary the line pressure to a “control” pressure value thatis supplied from an outlet of EVO valve 196 to an inlet of control valve198 via a third flow path 210. As seen, the low pressure fluid by-passedthrough EVO valve 196 flows through a fourth flow path 212 and is usedto cool and lubricate clutch pack 64. Fluid discharged from controlvalve 198 is returned via a fifth flow path 214 to sump 192. Finally,control valve 198 is shown to be in fluid communication with actuationchambers 132 via a sixth flow path 216 and in fluid communication withreturn chambers 134 via a seventh flow path 218. It will be appreciatedthat the components of the electrohydraulic control system can beintegrated into case assembly 78 to define a stand-alone assembly or, inthe alternative, be remotely located and connected via suitable hosesand tubing.

Rotary operator 92 is partially shown in FIG. 5 to illustrate itshydraulic connections with control valve 198. In the arrangement shown,EVO valve 196 is preferably an electromagnetic flow control valve thatis capable of variably regulating the fluid pressure delivered tocontrol valve 198. Likewise, control valve 198 is shown as a 3-positionshuttle valve. As seen, ECU 56 is operable to send electrical controlsignals to electric motor 194, EVO valve 196 and control valve 198. EVOvalve 196 is selectively actuated by ECU 56 to control the fluidpressure supplied to third flow path 210 while control valve 198 isselectively actuated to control delivery of the pressurized fluid toeither of actuation chambers 132 or return chambers 134. As best seenfrom FIGS. 3 and 3A, sixth flow path 216 is in fluid communication withannular chamber 140 via a flow passage 220 formed in case 78 whileseventh flow path 218 communicates with annular chamber 146 via anotherflow passage 222. A plurality of ring seals 224 are provided betweencase 78 and input shaft 42 to delimit chambers 140 and 146.

As contemplated by the present invention, ECU 56 is programmed toaccurately control the angular position of actuator ring 102 relative toreaction ring 100 based on a predefined control strategy fortransferring the desired amount of drive torque across transfer clutch46. The control strategy functions to determine and generate theelectric control signals sent to EVO valve 196 and control valve 198based on the mode signal from mode selector 54 and the sensor signalsfrom sensors 52. In addition, pressure sensor 202 sends a signal to ECU56 that is indicative of the fluid pressure delivered through sixth flowpath 216 to actuation chambers 132. Likewise, temperature sensor 200sends a signal to ECU 56 that is indicative of the fluid temperature insump 192.

In operation, if the vehicle operator selects the two-wheel drive mode,EVO valve 196 is opened and control valve 198 is initially actuated tocause third flow path 210 to be placed in communication with seventhflow path 218 while sixth flow path 216 is placed in communication withfifth flow path 214, whereby actuation chambers 132 are vented to sump192 while fluid at control pressure is supplied to return chambers 134.This results in actuator ring 102 being forced to its low pressureposition relative to reaction ring 100 for releasing engagement oftransfer clutch 46. Thereafter, control valve 198 can be shifted intoits closed position with no fluid delivered to actuation chambers 132 orreturn chambers 134 since return spring 178 forcibly biases actuatorring 102 to remain in its low pressure position. In contrast, uponselection of the part-time four-wheel drive mode, EVO valve 196 isopened and control valve 198 is actuated to connect third flow path 210to sixth flow path 216 and also connect seventh flow path 218 to fifthflow path 214, whereby actuation chambers 132 are supplied with fluid atcontrol pressure and return chambers 134 are vented to sump 192. Thehigh pressure fluid supplied to actuation chambers 132 causes actuatorring 102 to move to its high pressure position relative to reaction ring100, whereby ball ramp unit 94 is fully energized for moving apply plate96 to its locked position for fully engaging transfer clutch 46. PRV 204functions to limit the maximum fluid pressure that can be delivered toactuation chambers 132 during part-time four-wheel drive operation,thereby providing a torque limiting feature to prevent damage to clutchpack 64.

When mode selector 54 indicates selection of the on-demand four-wheeldrive mode, ECU 56 actuates control valve 198 so as to connect thirdflow path 210 to sixth flow path 216 while also connecting seventh flowpath 218 to fifth flow path 214. As such, return chambers 134 are ventedand supply actuation chambers 132 are supplied with pressurized fluidfrom EVO valve 196. ECU 56 functions to adaptively control EVO valve 196so as to initially supply fluid at a predetermined relatively lowpressure to actuation chambers 132 that causes actuator ring 102 toindex slightly relative to reaction ring 100 in the first direction.This angular movements causes actuator ring 102 to move from its lowpressure position to an intermediate or “ready” position which, in turn,results in ball ramp unit 94 moving apply plate 96 from its releasedposition to a “stand-by” position. In the stand-by position, apply plate96 exerts a small clutch engagement force on clutch pack 64.Accordingly, a small amount of drive torque is delivered to pinion shaft44 through transfer clutch 46 in this adapt-ready condition. Thereafter,ECU 56 determines when and how much drive torque needs to be transferredto pinion shaft 44 based on the current tractive conditions and/oroperating characteristics of the motor vehicle, as detected by sensors52.

Sensors 52 detect such parameters as, for example, the rotary speed ofthe input and pinion shafts, the vehicle speed and/or acceleration, thetransmission gear, the on/off status of the brakes, the steering angle,the road conditions, etc. Such sensor signals are used by ECU 56 todetermine a desired output torque value utilizing a control logic schemethat is incorporated into ECU 56. In particular, the control logiccorrelates the desired torque value to a fluid pressure value to bedelivered to actuation chambers 132. Based on this desired pressurevalue, ECU 56 actively controls actuation of EVO valve 196 to generate acorresponding pressure level in actuation chambers 132. Pressure sensor202 provides ECU 56 with direct feedback as to the actual fluid pressurein actuation chambers 132 so as to permit precise control of clutchactuator 52.

In addition to adaptive on-demand torque control, the present inventionpermits automatic release of transfer clutch 46 in the event of an ABSbraking condition or during the occurrence of an over-temperaturecondition. Specifically, when ECU 56 is signaled that an ABS brakecondition occurs, control valve 198 is immediately actuated to connectreturn chambers 134 with EVO valve 196 while actuation chambers 132 arevented to sump 192. Also, EVO valve 196 is fully opened to send fullpressure to return chambers 134, thereby forcibly moving actuator ring102 in its second direction to its low pressure position for fullyreleasing transfer clutch 46. Moreover, if temperature sensor 200detects that the fluid temperature in sump 192 exceeds a predeterminedthreshold value, the same type of immediate release of transfer clutch46 will be commanded by ECU 56.

While the control scheme has been described based on an on-demand torquetransfer strategy, it is contemplated that a differential “mimic”control strategy can likewise be used. Specifically, the torquedistribution between input shaft 42 and pinion shaft 44 can becontrolled to normally maintain a predetermined rear/front ratio (i.e.,70:30, 50:50, etc.) so as to simulate the inter-axle torque splittingfeature typically provided by a center differential unit. This desiredtorque distribution can then be adaptively controlled to address losttraction at either set of wheels. Regardless of whether the controllogic scheme is based on an on-demand or a differential torque transferstrategy, accurate control of the fluid pressure delivered to actuationchambers 132 of rotary operator 92 provides the desired torque transfercharacteristics being established across transfer clutch 46.

It is contemplated that the 3-position directional control valve 198shown in FIG. 5 could easily be substituted with a 2-positiondirectional valve or any other known valve device capable of controllingthe direction of flow in the hydraulic circuit. Likewise, FIG. 6illustrates a modified arrangement for electrohydraulic control system50 where control valve 198 now receives the line pressure directly frompump 190 via flow path 208 and EVO valve 196 is functional to regulatethe fluid pressure within actuation chambers 132 by controlling thefluid pressure within flow path 216. Again, EVO valve 196 is normallyopen with the by-passed fluid delivered through flow path 212 to cooland lubricate clutch pack 64. In operation, the fluid pressure suppliedto actuation chambers 132 is increased by closing EVO valve 196. Assuch, variable control of EVO valve 156 again results in variablepressure control within actuation chambers 132.

FIG. 7 is a hydraulic schematic of an electrohydraulic clutch system 50which is generally similar to FIG. 6 except that control valve 198 hasbeen eliminated. As such, ECU 56 controls the pumping direction of pump190 by reversing the polarity of motor 194. Check valves 230 and 232 areprovided to permit such a bi-directional pumping action. As before, thepressure profile of the fluid delivered to actuation chambers 132 isadaptively controlled via variable actuation of EVO valve 196.Optionally, EVO valve 196 can be eliminated with the pressure profilewithin actuation chambers 132 and return chambers 134 directlycontrolled by variable control of the rotary speed (RPM) and directionof electric motor 194.

The previous hydraulic circuits utilized control valve 198 fordirectional control in conjunction with EVO valve 196 for adaptivepressure regulation. As an alternative, FIG. 8 illustrates a hydrauliccircuit wherein a proportional control valve 234 is used to regulateboth the directional and pressure characteristics. Preferably,proportional control valve 234 is a pulse width modulated (PWM) valvehaving a moveable element that is controlled by an electromagneticsolenoid based on electric control signals from ECU 56. In addition, aflow control valve 236 is provided in flow path 208 to preventdead-heading of pump 190 and to provide cooling flow to clutch pack 64.

The arrangement shown for drive axle assembly 34 of FIG. 2 is operableto provide the on-demand four-wheel drive mode by adaptivelytransferring drive torque from primary driveline 14 (via propshaft 28)to secondary driveline (via pinion shaft 44). In contrast, a drive axleassembly 34A is shown in FIG. 9 with torque coupling 38 now installedbetween differential case 70 and one of axleshafts 30 to provide anadaptive system for biasing the torque and limiting intra-axle slipbetween the rear wheels 32. As before, torque coupling 38 isschematically shown to include transfer clutch 46 and clutch actuator48, the construction and function of which are understood to be similarto the detailed description previously provided herein for eachsub-assembly. It will be understood that this particular “limited slip”differential arrangement can either be used in association with theon-demand drive axle assembly shown in FIG. 2 or in association with adrive axle assembly wherein propshaft 28 directly drives differentialunit 40.

Referring now to FIG. 10, a drive axle assembly 34B is schematicallyshown to include a pair of torque couplings 38L and 38R that areoperably installed between a driven shaft 44 or 28 and axleshafts 30.The driven pinion shaft drives a right-angled gearset including pinion66 and ring gear 68 which, in turn, drives a transfer shaft 240. Firsttorque coupling 38L is shown disposed between transfer shaft 240 and theleft axleshafts 30 while second torque coupling 38R is disposed betweentransfer shaft 240 and the right axle shaft 30. Each coupling includes acorresponding transfer clutch 46L, 46R and a clutch actuator 48L, 48R.Accordingly, independent slip control between the driven pinion shaftand each wheel 32 is provided by this arrangement. A common sump isprovided with ECU 56 controlling independent actuation of both clutchactuators 48L and 48R.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 11 schematically depicts afront-wheel based four-wheel drive drivetrain layout 10′ for a motorvehicle. In particular, engine 18 drives a multi-speed transaxle 20having an integrated front differential unit 25 for driving front wheels22 via axle shafts 24. As before, PTU 26 is also driven by transaxle 20for delivering drive torque to the input member of a torque coupling238. The output member of torque coupling 238 is coupled to propshaft 28which, in turn, drives rear wheels 32 via axle assembly 34. Rear axleassembly 34 can be a traditional driven axle with a differential or, inthe alternative, be similar to the arrangements described in associationwith FIG. 9 or 10. Accordingly, in response to the detection of a frontwheel slip condition, torque coupling unit 238 is adaptively actuated todeliver drive torque “on-demand” to rear wheels 32. Again, it iscontemplated that torque coupling unit 238 is substantially similar instructure and function to that of torque coupling unit 38 previouslydescribed herein.

Referring now to FIG. 12, torque coupling 238 is schematicallyillustrated in association with an on-demand four-wheel drive systembased on a front-whell drive vehicle similar to that shown in FIG. 11.In particular, an output shaft 240 of transaxle 20 is shown to drive anoutput gear 242 which, in turn, drives an input gear 244 that is fixedto a carrier 246 associated with front differential unit 25. To providedrive torque to front wheels 22, front differential unit 25 includes apair of side gears 248 that are connected to front wheels 22 viaaxleshafts 24. Differential unit 25 also includes pinions 250 that arerotatably supported on pinion shafts fixed to carrier 246 and which aremeshed with side gears 248. A transfer shaft 252 is provided fortransferring drive torque from carrier 246 to a clutch hub 62 associatedwith transfer clutch 46. PTU 26 is a right-angled drive mechanismincluding a ring gear 254 fixed for rotation with drum 60 of transferclutch 46 and which is meshed with a pinion gear 256 fixed for rotationwith propshaft 28. According to the present invention, the componentsschematically shown for torque transfer mechanism 238 are understood tobe similar to those previously described. In operation, the powertransfer system permits drive torque to be adaptively transferred fromthe primary (i.e., front) driveline to the secondary (i.e., rear)driveline.

Referring to FIG. 13, a modified version of the power transmissiondevice shown in FIG. 12 now includes a second torque coupling 238A thatis arranged to provide a limited slip feature in association withprimary differential 25. As before, torque coupling 238 provideson-demand transfer of drive torque from the primary driveline to thesecondary driveline. In addition, second torque coupling 238A nowprovides on-demand torque biasing (side-to-side) between axleshafts 24of primary driveline 14.

FIG. 14 illustrates another modified version of FIG. 12 wherein anon-demand four-wheel drive system is shown based on a rear-wheel drivemotor vehicle that is arranged to normally deliver drive torque to rearwheels 32 while selectively transmitting drive torque to front wheels 22through a torque coupling 238. In this arrangement, drive torque istransmitted directly from transmission output shaft 240 to powertransfer unit 26 via a drive shaft 260 which interconnects input gear244 to ring gear 254. To provide drive torque to front wheels 22, torquecoupling 238 is shown operably disposed between drive shaft 260 andtransfer shaft 252. In particular, transfer clutch 46 is arranged suchthat drum 60 is driven with ring gear 254 by drive shaft 260. As such,clutch actuator 48 can be adaptively actuated to transfer drive torquefrom drum 60 through clutch pack 64 to hub 62 which, in turn, drivescarrier 246 of front differential unit 25 via transfer shaft 252.

In addition to the on-demand four-wheel drive systems shown previously,the power transmission technology of the present invention can likewisebe used in full-time four-wheel drive systems to adaptively bias thetorque distribution transmitted by a center or “interaxle” differentialunit to the front and rear drivelines. For example, FIG. 15schematically illustrates a full-time four-wheel drive system which isgenerally similar to the on-demand four-wheel drive system shown in FIG.14 with the exception that an interaxle differential unit 270 is nowoperably installed between carrier 246 of front differential unit 25 andtransfer shaft 252. In particular, output gear 244 is fixed for rotationwith a carrier 272 of interaxle differential 270 from which pinion gears274 are rotatably supported. A first side gear 276 is meshed with piniongears 274 and is fixed for rotation with drive shaft 260 so as to bedrivingly interconnected to the rear driveline through power transferunit 26. Likewise, a second side gear 278 is meshed with pinion gears274 and is fixed for rotation with carrier 246 of front differentialunit 25 so as to be drivingly interconnected to the front driveline.Torque transfer mechanism 238 is now shown to be operably disposedbetween side gears 276 and 278. Torque transfer mechanism 238 isoperably arranged between the driven outputs of interaxle differential270 for providing an adaptive torque biasing and slip limiting function.

Referring now to FIG. 16, a drivetrain for a four-wheel drive vehicle isshown to include engine 18, a multi-speed transmission 20′ fordelivering drive torque to a primary or rear driveline 16′ through apower transmission device, thereinafter referred to as transfer case300. As seen, transfer case 300 has a rear output shaft 302interconnected between the output of transmission 20′ and a rearpropshaft 28′. Further, propshaft 28′ is shown to drive a pinion shaft44′ for driving a drive axle assembly which, in this example, is similarto rear axle assembly 34A of FIG. 9. A secondary or front driveline 14′includes a front propshaft 304 interconnecting a front output shaft 306of transfer case 300 to a conventional front axle assembly 308. Atransfer assembly associated with transfer case 300 includes a firstsprocket 310 rotatably supported on rear output shaft 302, a secondsprocket 312 fixed to front output shaft 306, and a chain 314 enmeshedtherebetween. Transfer case 300 is shown to include a torque coupling 38for providing on-demand transfer of drive torque from rear output shaft302 through the transfer assembly to first output shaft 306. As seen,transfer case 300 has an electrohydraulic control system 50′ that iscontrolled by ECU 56 in coordination with electrohydraulic controlsystem 50 associated with torque coupling 38 in drive axle 34A.

Referring now to FIG. 17, a full-time 4WD system is shown to include atransfer case 300′ which is generally similar to transfer case 300 ofFIG. 16 except that an interaxle differential 320 is provided between aninput shaft 322 and output shafts 302 and 306. As is conventional, inputshaft 322 is driven by the output of transmission 20′. Differential 320includes an input defined as a planet carrier 324, a first outputdefined as a first sun gear 326, a second output defined as a second sungear 328, and a gearset for permitting speed differentiation betweenfirst and second sun gears 326 and 328. The gearset includes a pluralityof meshed pairs of first planet gears 330 and second pinions 332 whichare rotatably supported by carrier 324. First planet gears 330 are shownto mesh with first sun gear 326 while second planet gears 332 are meshedwith second sun gear 328. First sun gear 326 is fixed for rotation withrear output shaft 302 so as to transmit drive torque to rear driveline16′. To transmit drive torque to front driveline 14′, second sun gear328 is coupled to the transfer assembly which again includes firstsprocket 310 rotatably supported on rear output shaft 302, secondsprocket 312 fixed to front output shaft 306, and power chain 314.

A number of preferred embodiments have been disclosed to provide thoseskilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the presentinvention. The invention being thus described, it will be obvious thatvarious modifications can be made without departing from the true spiritand scope of the invention, and all such modifications as would beconsidered by those skilled in the art are intended to be includedwithin the scope of the following claims.

1. A power transmission device comprising: a rotary input member adaptedto receive drive torque from a source of drive torque; a rotary outputmember adapted to transmit drive torque to an output device; a torquetransmission mechanism operable for transferring drive torque from saidinput member to said output member, said torque transmission mechanismincluding a friction clutch operably disposed between said input memberand said output member and a clutch actuator for controlling engagementof said friction clutch, said clutch actuator including a rotaryoperator and a thrust mechanism, said rotary operator having first andsecond components defining an actuation chamber and a return chambertherebetween, said first component being fixed for rotation with one ofsaid input and output members and said second component adapted torotate relative to said first component in response to a fluid pressuredifferential between said actuation and return chambers, said thrustmechanism operable for applying a clutch engagement force on saidfriction clutch in response to rotation of said second componentrelative to said first component; and a fluid control system forregulating said fluid pressure differential between said actuation andreturn chambers.
 2. The power transmission device of claim 1 whereinsaid fluid control system includes a source of hydraulic fluid providingfluid to a pump and a control valve disposed in a hydraulic circuitbetween said pump and at least one of said actuation and returnchambers.
 3. The power transmission device of claim 2 wherein said fluidcontrol system further includes a control unit operable to controlactuation of said control valve for varying the magnitude of the fluidpressure supplied to said actuation chamber as a function of a rotaryspeed difference between said input and output members.
 4. The powertransmission device of claim 3 wherein said fluid control system furtherincludes a pressure sensor which provides a signal to said control unitthat is indicative of the value of the fluid pressure in said actuationchamber.
 5. The power transmission device of claim 1 wherein angularmovement of said second component to a low pressure position relative tosaid first component causes said thrust mechanism to be located in afirst position for applying a minimum clutch engagement force on saidfriction clutch, and wherein angular movement of said second componentto a high pressure position relative to said first component causes saidthrust mechanism to move to a second position for applying a maximumclutch engagement force on said friction clutch, said second componentis moveable between its low pressure and high pressure positions due tothe magnitude of the fluid pressure delivered to at least one of saidactuation chamber and said return chamber by said fluid control system.6. The power transmission device of claim 1 wherein said first componentof said rotary operator is a first ring having a first body segment andplurality of first lugs so as to define a plurality of channelstherebetween, and wherein said second component of said rotary actuatoris a second ring having a second body segment and a plurality of secondlugs which extend into said channels so as to define an alternatingseries of said actuation chambers and said return chambers.
 7. The powertransmission device of claim 6 wherein said second ring is fixed to adrive component of said thrust mechanism such that rotation of saiddrive component results in translational movement of a driven componentof said thrust mechanism for controlling the magnitude of said clutchengagement force applied to said friction clutch.
 8. The powertransmission device of claim 7 wherein said thrust mechanism is a ballramp unit with a first cam member as its drive component, a second cammember as its driven component, and rollers engaging a cam surfaceformed between said first and second cam members, wherein said camsurface is configured to cause translational movement of said second cammember in response to rotary movement of said first cam member, andwherein said second cam member is arranged to cause correspondingmovement of an apply plate relative to said friction clutch.
 9. Thepower transmission device of claim 8 wherein an increase in fluidpressure in said actuation chambers and a reduction in fluid pressure insaid return chambers causes said second ring and said first cam memberto rotate in a first direction relative to said first ring for causingcorresponding movement of said second cam member from a first positiontoward a second position for axially moving said apply plate from areleased position toward a locked position relative to said frictionclutch.
 10. The power transmission device of claim 9 wherein a decreasein fluid pressure in said actuation chambers and an increase in fluidpressure in said return chambers causes said second ring and said firstcam member to rotate in a second direction relative to said first ringfor causing movement of said second cam member toward its first positionfor axially moving said apply plate toward its released position. 11.The power transmission device of claim 1 wherein said input member is afirst shaft in a transfer case and said output member is a second shaftof said transfer case.
 12. The power transmission device of claim 1wherein said input member is driven by a powertrain of a motor vehicleand said output member is connected to a differential unit of a driveaxle assembly.
 13. The power transmission device of claim 1 defining adrive axle assembly having a differential unit interconnecting a pair ofaxleshafts, and wherein said input member is a differential carrier ofsaid differential unit, said output member is one of said axleshafts,and said torque transmission unit is arranged to adaptively limit slipbetween said axleshafts.
 14. The power transmission device of claim 1wherein said fluid control system includes a first flow path supplyingfluid from a fluid source to a fluid pump, a second flow path supplyingfluid from said pump to a control valve, a third flow path connectingsaid control valve to said actuation chamber, and a fourth flow pathconnecting said control valve to said return chamber.
 15. The powertransfer device of claim 14 wherein said control valve is operable in afirst mode to deliver pressurized fluid through said third flow path tosaid actuation chamber and vent fluid from said return chamber throughsaid fourth flow path so as to cause said second component to rotate ina first direction relative to said first component from a low pressureposition toward a high pressure position for causing said thrustmechanism to increase said clutch engagement force exerted on saidfriction clutch, and wherein said control valve is operable in a secondmode to deliver pressurized fluid through said fourth flow path to saidreturn chamber and vent fluid from said actuation chamber through saidthird flow path so as to cause said second component to rotate in asecond direction relative to said first component toward its lowpressure position for causing said thrust mechanism to decrease saidclutch engagement force exerted on said friction clutch.
 16. The powertransmission device of claim 14 wherein said fluid control systemfurther includes a second control valve disposed in said second flowpath between said pump and said first control valve, and wherein saidsecond control valve is selectively actuated to vary the fluid pressuresupplied to said actuation chamber for controlling the angular positionof said second component of said rotary operator relative to said firstcomponent.
 17. The power transmission device of claim 14 wherein saidfluid control system further includes a second control valve disposed insaid third flow path between said first control valve and said actuationchamber, and wherein said second control valve is actuated for varyingthe fluid pressure supplied to said actuation chamber so as to controlthe angular position of said second component relative to said firstcomponent.
 18. The power transmission device of claim 1 wherein saidfluid control system includes a first flow path supplying fluid from afluid source to a pump, a second flow path connecting said pump to saidactuation chamber, and a third flow path connecting said pump to saidreturn chamber, and wherein said control valve is disposed in saidsecond path and is operable for regulating the fluid pressure withinsaid actuation chamber so as to control the angular position of saidsecond component relative to said first component for varying themagnitude of said clutch engagement force exerted by said thrustmechanism on said friction clutch.
 19. A power transfer device for usein a motor vehicle having a powertrain and first and second drivelines,comprising: a first shaft driven by the powertrain and adapted forconnection to the first driveline; a second shaft adapted for connectionto the second driveline; a torque transmission mechanism fortransferring drive torque from said first shaft to said second shaft,said torque transmission mechanism including a friction clutch operablydisposed between said first shaft and said second shaft, and a clutchactuator for engaging said friction clutch, said clutch actuatorincluding a rotary operator and a thrust mechanism, said rotary operatorhaving first and second components which define an actuation chamber anda return chamber, said first component being fixed for rotation with oneof said first and second shafts and said second component adapted torotate relative to said first component in response to a fluid pressuredifferential between said actuation and return chambers, said thrustmechanism operable for applying a clutch engagement force to saidfriction clutch in response to rotation of said second componentrelative to said first component; and a control system for regulatingsaid fluid pressure differential between said actuation and returnchambers.
 20. The power transmission device of claim 19 wherein saidcontrol system includes a source of hydraulic fluid providing fluid to apump and a control valve disposed in a hydraulic circuit between saidpump and at least one of said actuation and return chambers.
 21. Thepower transfer device of claim 20 wherein said control system furtherincludes a control unit is operable to control actuation of said controlvalve for adaptively varying the magnitude of the fluid pressuresupplied to said actuation chamber as a function of a rotary speeddifference between said first and second shafts.
 22. The power transferdevice of claim 21 wherein said control system further includes apressure sensor which provides a signal to said control unit that isindicative of the value of the fluid pressure in said actuation chamber.23. The power transfer device of claim 19 wherein angular movement ofsaid second component to a low pressure position relative to said firstcomponent causes said thrust mechanism to be located in a first positionfor applying a minimum clutch engagement force on said friction clutch,wherein angular movement of said second component to a high pressureposition relative to said first component causes said thrust mechanismto move to a second position for applying a maximum clutch engagementforce on said friction clutch, and wherein said second component ismoveable between its low pressure and high pressure positions due to themagnitude of said fluid pressure differential between said actuation andreturn chambers.
 24. The power transfer device of claim 19 wherein saidfirst component of said rotary operator is a first ring having a firstbody segment and a plurality of radially extending first lugs whichdefine a series of channels therebetween, wherein said second componentis a second ring having a second body segment and a plurality ofradially extending second lugs which extend into said channels so as todefine a plurality of said actuation chambers and return chambers,wherein said actuator chambers are in fluid communication with an outletof a control valve, and wherein a fluid pump is operable to draw fluidfrom a fluid source and deliver high pressure fluid to said controlvalve such that selective control of said control valve results inrotary movement of said second ring relative to said first ring.
 25. Thepower transfer device of claim 24 wherein said second ring is fixed to adrive component of said thrust mechanism such that rotation of saiddrive component results in translational movement of a driven componentof said thrust mechanism for exerting said clutch engagement force onsaid friction clutch.
 26. The power transfer device of claim 25 whereinsaid thrust mechanism is a ball ramp unit with a first cam member as itsdrive component, a second cam member as its driven component, androllers engage a cam surface formed between said first and second cammember, and wherein said cam surface is configured to causetranslational movement of said second cam member in response to rotarymovement of said first cam member for applying said clutch engagementforce to said friction clutch.
 27. The power transfer device of claim 20wherein said hydraulic circuit includes a first flow path supplyingfluid from said fluid source to said pump, a second flow path supplyingfluid from said pump to said control valve, a third flow path connectingsaid control valve to said actuation chamber and a fourth flow pathconnecting said control valve to said return chamber.
 28. The powertransfer device of claim 27 wherein said control valve is operable in afirst mode to deliver pressurized fluid through said third flow path tosaid actuation chamber and vent fluid from said return chamber throughsaid fourth flow path so as to cause said second component to rotate ina first direction relative to said first component from a low pressureposition toward a high pressure position for causing said thrustmechanism to increase the clutch engagement force exerted on saidfriction clutch, and wherein said control valve is operable in a secondmode to deliver pressurized fluid through said fourth flow path to saidreturn chamber and vent fluid from said actuation chamber through saidthird flow path so as to cause said second component to rotate in asecond direction relative to said first component toward its lowpressure position for causing said thrust mechanism to decrease theclutch engagement force exerted on said friction clutch.
 29. The powertransfer device of claim 27 wherein said control system further includesa second control valve disposed in said second flow path between saidpump and said first control valve, and wherein said second control valveis selectively actuated to vary the fluid pressure supplied to saidactuation chamber for controlling the angular position of said secondcomponent of said rotary operator relative to said first component. 30.The power transfer device of claim 27 wherein said control systemfurther includes a second control valve disposed in said third flow pathbetween said first control valve and said actuation chamber, and whereinsaid second control valve is actuated for varying the fluid pressuresupplied to said actuation chamber so as to control the angular positionof said second component relative to said first component.
 31. The powertransfer device of claim 20 wherein said hydraulic circuit includes afirst flow path supplying fluid from a fluid source to said pump, asecond flow path connecting said pump to said actuation chamber, and athird flow path connecting said pump to said return chamber, and whereinsaid control valve is disposed in said second path and is operable forregulating the fluid pressure within said actuation chamber so as tocontrol the angular position of said second component relative to saidfirst component of said rotary actuator for varying the magnitude ofsaid clutch engagement force exerted by said thrust mechanism on saidfriction clutch.
 32. A torque transmission mechanism for use in a motorvehicle having a powertrain and a driveline, comprising: an input memberdriven by the powertrain; an output member driving the driveline; aclutch pack operably disposed between said input and output members; anapply plate moveable relative to said clutch pack between a firstposition and a second position, said apply plate is operable in itsfirst position to apply a minimum clutch engagement force on said clutchpack and said apply plate is operable in its second position to apply amaximum clutch engagement force on said clutch pack; a clutch actuatorfor controlling movement of said apply plate between its first andsecond positions, said clutch actuator including a rotary actuatorhaving first and second components that are coaxially arranged to definea first chamber and a second chamber therebetween, said first componentof said rotary operator is fixed for rotation with one of said input andoutput members and said second component is adapted to rotate relativeto said first component in response to a fluid pressure differentialbetween said first and second chambers, and said apply plate is operableto move between its first and second positions in response to rotationof said second component relative to said first component; and a controlsystem including a control valve disposed in a hydraulic circuit betweensaid first and second chambers and a control unit for controllingactuation of said control valve for regulating said fluid pressuredifferential between said first and second chambers.
 33. The powertransmission device of claim 32 wherein said control unit is operable tovary the magnitude of the fluid pressure supplied to one of said firstand second chambers as a function of a rotary speed difference betweensaid input and output members.
 34. The torque transmission unit of claim33 wherein angular movement of said second component to a first positionrelative to said first component causes said apply plate to move to itsfirst position, wherein angular movement of said second component to asecond position relative to said first component causes said apply plateto move to its second position, and wherein movement of said secondcomponent from its first position toward its second position is causedby an increase in the fluid pressure delivered by said control valve tosaid first chamber.
 35. The torque transmission unit of claim 33 whereinsaid first component of said rotary operator is a first ring having afirst body segment and plurality of first lugs so as to define aplurality of channels therebetween, and wherein said second component ofsaid rotary actuator is a second ring having a second body segment and aplurality of second lugs which extend into said channels so as to definean alternating series of first and second chambers between adjacentpairs of said first lugs.
 36. The torque transmission unit of claim 35wherein said second ring is fixed to a drive component of a thrustmechanism such that rotation of said drive component results intranslational movement of a driven component of said thrust mechanismfor controlling the magnitude of said clutch engagement force applied bysaid apply plate to said friction clutch.
 37. The torque transmissionunit of claim 36 wherein said thrust mechanism is a ball ramp unit witha first cam member as its drive component, a second cam member as itsdriven component, and rollers engaging a cam surface between said firstand second cam members, wherein said cam surface is configured to causetranslational movement of said second cam member in response to rotarymovement of said first cam member, and wherein said second cam member isarranged to cause corresponding translational movement of said applyplate relative to said friction clutch.
 38. The torque transmission unitof claim 37 wherein an increase in fluid pressure in said first chambersand a reduction in fluid pressure in said second chambers causes saidsecond ring and said first cam member to rotate in a first directionrelative to said first ring for causing said second cam member toaxially move said apply plate from its first position toward its secondposition relative to said friction clutch, and wherein a decrease influid pressure in said first chambers and an increase in fluid pressurein said second chambers causes said second ring and said first cammember to rotate in a second direction relative to said first ring forcausing said second cam member to axially move said apply plate towardits first position.
 39. The torque transmission unit of claim 32 whereinsaid hydraulic circuit includes a first flow path supplying fluid from afluid source to a pump, a second flow path supplying fluid from saidpump to said control valve, a third flow path connecting said controlvalve to said first chamber, and a fourth flow path connecting saidcontrol valve to said second chamber.
 40. The torque transmission unitof claim 39 wherein said control system further includes a secondcontrol valve disposed in said second flow path between said pump andsaid first control valve, and wherein said second control valve isselectively actuated by said control unit to vary the fluid pressuresupplied to said first chamber for controlling the angular position ofsaid second component of said rotary operator relative to said firstcomponent.
 41. The torque transmission unit of claim 39 wherein saidcontrol system further includes a second control valve disposed in saidthird flow path between said first control valve and said first chamber,and wherein said second control valve is actuated by said control unitfor varying the fluid pressure supplied to said first chamber so as tocontrol the angular position of said second component relative to saidfirst component.
 42. The torque transmission unit of claim 32 whereinsaid hydraulic circuit includes a first flow path supplying fluid from afluid source to a pump, a second flow path connecting said pump to saidfirst chamber, and a third flow path connecting said pump to said secondchamber, and wherein said control valve is disposed in said second pathand is operable for regulating the fluid pressure within said firstchamber so as to control the angular position of said second componentrelative to said first component for varying the magnitude of clutchengagement force exerted on said friction clutch.